1. Field of the Invention
This invention relates to Er3+ doped tellurite glasses and more specifically to Er3+ doped boro-tellurite glasses with increased phonon energy for 1.5 μm broadband amplification.
2. Description of the Related Art
Optical amplifiers are considered enabling components for bandwidth expansion in fiber optic communications systems. In particular, silica glass erbium doped fiber amplifiers (EDFA) exhibit many desirable attributes including high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fiber amplifiers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require conversion from electrical energy to photon energy. In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data in route between a source and destination. To function properly, the amplifiers must exhibit high small signal gains and/or high output saturation powers over a desired bandwidth. One drawback of silica EDFAs is their limited 30 nm bandwidth, which limits the transmission capacity of WDM systems.
Tellurite glasses provide a broad bandwidth of over 70 nm and thus have received considerable attention for use in EDFAs. See A. Mori et al “1.5 μm Broadband Amplification By Tellurite-Based EDFAs,” Technical Digest of Conf. Optical Fibe-Comm. 1997 (OFC′97), Feb 16-21, 1997 and Y. Ohishi et al. “Gain Characteristics of Tellurite-Based Erbium-Doped Fiber Amplifiers for 1.5 μm Broadband Amplification” Opt. Lett., vol. 23, no. 4, 1998, p. 274.
To amplify a 1.5 μm signal, EDFAs can be optically pumped at 1480 nm or at 980 nm as shown in the energy level diagram 10 of Er3+, FIG. 1. Pumping at 1480 nm is typically used for high power EDFAs because the ground state absorption to the 4I13/2 energy level has a high absorption cross-section relative to the 4I11/2 energy level. Unfortunately, this scheme does not provide full population inversion or good SNR and is not adequate for many EDFA applications.
980 nm optical pumping provides good SNR and low cost but the small signal gain is significantly less than what is achieved with 1480 nm pumping for erbium doped low phonon energy glass fibers, such as fluorite glass fiber and tellurite glass fiber. Tellurite glasses contain heavy elements, which translates into small phonon energy (typically between 680 and 785 cm−1) as compared to silicate glasses which present high phonon energy (typically around 1100 cm−1). Phonon energy has a strong influence on the lifetimes of the different excited states of Er3+ because the relaxation between levels is dominated by multiphonon processes. The larger the number of phonons involved, the smaller the probability of relaxation to the lower energy level, and the longer the lifetime of a given excited state, for instance 4I11/2 of erbium ions.
With 980 nm pumping the level 4I11/2 gets populated first, and then through phonon-assisted relaxation the lower level 4I13/2 gets populated. Gain is achieved through transition between the levels 4I13/2 and 4I15/2. For optimal operation, the lifetime of the level 4I11/2 should be as short as possible. Otherwise, excited state absorption processes from the level 4I11/2 to higher energy excited states such as 4F7/2 will occur and reduce the gain at 1550 nm. Consequently, the low phonon energy of tellurite glass creates longer lifetimes, which in turn reduces small signal gain when pumped with 980 nm laser diode.
Y. G. Choi et al, “Enhanced 4I11/2→4I13/2 Transition Rate in Er3+/Ce3+-Codoped Tellurite Glasses,” Electron. Lett. Vol. 35, no. 20, 1999, p. 1765 proposed Ce3+-codoping to enhance the 980 nm pumping efficiency through the non-radiative energy transfer Er3+:4I11/2, Ce3+:2F5/2→Er3+:4I13/2, Ce3+: 2F7/2. The co-doping provides an additional channel for the relaxation 4I11/2→4I13/2 in the Er3+-doped tellurite glasses which shortens the lifetime of the 4I11/2 level and enhances the population accumulation in the 4I13/2 level and the 980 nm pumping efficiency.
A more effective approach would be to increase the phonon energy of the tellurite glass without sacrificing the glass' optical, thermal stability or chemical durability properties.